【模板】【凸包】Graham_scan

 /*
 唐代李白
 《江夏别宋之悌》
 楚水清若空，遥将碧海通。人分千里外，兴在一杯中。
 谷鸟吟晴日，江猿啸晚风。平生不下泪，于此泣无穷.
 */
 #include <iostream>
 #include <cstdio>
 #include <algorithm>
 #include <cstring>
 #include <vector>
 #include <utility>
 #include <iomanip>
 #include <string>
 #include <cmath>
 #include <queue>
 #include <assert.h>
 #include <map>
 #include <ctime>
 #include <cstdlib>
 #include <stack>
 #include <set>
 #define LOCAL
 const int INF = 0x7fffffff;
 const int MAXN =   + ;
 const int maxnode =  *  +  * ;
 const int MAXM =  + ;
 const int MAX = ;
 using namespace std;
 struct point{//也是vector
        int x, y;
 }p[MAXN], stack[MAXN];
 int top;
 int dis(point p1,point p2){return (p1.x-p2.x)*(p1.x-p2.x)+(p1.y-p2.y)*(p1.y-p2.y);}
 //叉积，结果小于表示向量p0p1的极角大于p0p2的极角，等于则0两向量共线
 int  multi(point p1, point p2, point p0){//p0为坐标
     return (p1.x - p0.x) * (p2.y - p0.y) - (p2.x - p0.x) * (p1.y - p0.y);
 }

 int cmp(point a,point b){
     if(multi(a,b,p[]) > ) return ;
     if(multi(a,b,p[]) ==  && dis(a,p[]) < dis(b,p[])) return ;
     return ;
 }
 //Graham_scan凸包扫描
 void Graham_scan(point p[],point stack[],int n){
      int i, j, k = ;
      top=;
      point temp;

      for(i=;i<n;i++) if(p[i].y<p[k].y||((p[i].y==p[k].y)&&(p[i].x<p[k].x))) k=i;
      swap(p[], p[k]);
      //按极角从小到大，距离偏短进行排序
      sort(p + , p + n, cmp);
      stack[] = p[];
      stack[] = p[];
      stack[] = p[];
      for (int i = ; i < n; i++){
          while(top >  && multi(p[i],stack[top],stack[top-]) >= )
          top--;
          stack[++top] = p[i];
      }
 }

 int main(){
    #ifdef LOCAL
    freopen("data.txt", "r", stdin);
    freopen("out.txt", "w", stdout);
    #endif
    //printf("%d", (a == c));
    return ;
 }